Solid state ceramic microwave heating susceptor

ABSTRACT

Disclosed are ceramic compositions which are useful in the formulation and fabrication of microwave susceptors for disposable packages for the microwave heating of food items. The compositions include a novel microwave absorbing material and a binder. The novel microwave absorbing materials comprise ceramics with neutral lattice charges such as clays, kaolin, talc, silicates, alumina, aluminosilicates and mixtures thereof. The compositions provide good heat generation and a predeterminable upper temperature limit. The materials are common and inexpensive. Preferred compositions additionally include a temperature profile moderator which can be common salt.

BACKGROUND OF THE INVENTION

1. The Technical Field

This invention relates generally to the art of the microwave heating byhigh frequency electromagnetic radiation or microwave energy. Moreparticularly, the present invention relates to ceramic compositionsuseful for fabrication in or into microwave heating susceptors,especially for disposable microwave packages for food products.

2. Background Art

The heating of food articles with microwave energy by consumers has nowbecome commonplace. Such microwave heating provides the advantages ofspeed and convenience. However, heating breaded food with microwavesoften gives them a soggy texture and fails to impart the desirablebrowning flavor and/or crispness of conventionally oven heated productsdue in part to retention of oil and moisture. Unfortunately, ifmicrowave heating is continued in an attempt to obtain a crisp exterior,the interior is generally overheated or overdone. Moreover the microwavefields in the ovens are uneven which can lead to unevenness or both hotand cold spots within food items or packaged food items being heated.

The prior art includes many attempts to overcome such disadvantageswhile attempting to retain the advantages of microwave heating. That is,the prior art includes attempts at providing browning or searing meansin addition to microwave heating. Basically, three approaches existwhether employing permanent dishes or disposable packages to providingmicrowave heating elements which provide such browning or searing andwhich elements are referred to herein and sometimes in the art asmicrowave heating susceptors. In the art, materials which are microwaveabsorptive are referred to as "lossy" while materials which are not arereferred to as "non-lossy" or, equivalently, merely "transparent."

The first approach is to include an electrically resistive film usuallyquite thin, e.g., 0.00001 to 0.00002 cm., applied to the surface of anon-conductor or non-lossy substrate. In the case of a permanent dish,the container is frequently ceramic while for a disposable package thesubstrate can be a polyester film. Heat is produced because of the I² Ror resistive loss (see for example, U.S. Pat. Nos. 3,853,612, 3,705,054,3,922,452 and 3,783,220). Examples of disposable packaging materialsinclude metallized films such as described in U.S. Pat. Nos. 4,594,492,4,592,914, 4,590,349, 4,267,420 and 4,230,924.

A second category of microwave absorbing materials comprise electricconductors such as parallel rods, cups or strips which function toproduce an intense fringing electric field pattern that causes surfaceheating in an adjacent food. Examples include U.S. Pat. Nos. 2,540,036,3,271,552, 3,591,751, 3,857,009, 3,946,187 and 3,946,188. Such anapproach is usually taken with reusable utensils or dishes.

A third approach is to form articles from a mass or bed of particlesthat become hot in bulk when exposed to microwave energy. The microwaveabsorbing substance can be composed of ferrites, carbon particles, etc.Examples of such compositions or articles prepared therefrom include,for example, U.S. Pat. Nos. 2,582,174, 2,830,162 and 4,190,757.

A review of the prior art, especially that art directed towardsprovision of heating susceptors for disposable packages for microwaveheating of foods indicates at least three basic problems exist in theformulation and fabrication of heating susceptors. One difficulty withthe third category of materials, generally, is that they can exhibitrunaway heating, that is, upon further microwave heating theirtemperature continues to increase. Great care must be taken infabrication of safe articles containing such materials. Metallized filmmaterials of the first category can be formulated and fabricated suchthat they do not exhibit runaway heating. However, such films sufferfrom the second problem; namely that while their operating temperaturesare quite hot, are at controlled temperature, and are sufficient tobrown the surface of nearby food items, due to their thinness and littlemass, only small quantities of heat are actually generated. Suchmaterials are thus unsuitable for certain foods which require absorptionof great amounts of heat in their preparation, e.g., cake batters. Thethird general problem is one of cost. Microwave susceptors frequentlycomprise costly materials. Also, fabrication of susceptor structuresfrequently is complex and expensive.

Accordingly, in view of the above-noted problems with present microwavesusceptors, an object of the present invention is to provide a devicewhich will heat under the influence of the microwave radiation up to anupper temperature limit at which temperatures the device comes to asteady state absorption of microwave energy and heating to a highertemperature is precluded.

Another object of the present invention is to provide a microwaveheating device or susceptor which is disposable and adapted for use withpre-prepared foods.

A still further object of the present invention is to provide a heatingdevice which can be utilized as a non-disposable utensil or tray.

A still further object of the present invention is to provide a heatingdevice which by appropriate selection of manufacturing parameters canprovide a predetermined upper temperature limit and moderate microwaveheating of the food item through absorption and moderation of themicrowave energy.

Another object of the present invention is to provide a heating deviceor utensil which is inexpensive to manufacture, safe to use and welladapted for its intended use.

Surprisingly, the above objectives can be realized and new compositionsprovided which overcome the problems associated with previous materialswhich have been used for the fabrication of microwave heatingsusceptors. The present compositions do not exhibit runaway heating yetgenerate relatively large amounts of heat. Indeed, the final heatingtemperature can be controlled quite closely. Also, the presentcompositions are comprised of materials which are commonly available andinexpensive. In the most surprising aspect of the present invention, thecompositions comprise ceramic materials previously considered alone tobe microwave transparent or used in microwave transparent ceramiccompositions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a packaged food article for microwaveheating constructed in accordance with the teachings of the invention;

FIG. 2 is a perspective view of the packaged food article with outerpaperboard outerwrap opened and with an inner tray and sleeve showndisengaged;

FIG. 3 is a perspective view of the tray disengaged from the sleeve andholding several food pieces;

FIG. 4 is a perspective view of the tray with the food items removedshowing a microwave heating susceptor raised above its resting positionin the tray;

FIG. 5 is a cross sectional view of the tray taken in the direction oflines 5--5 of FIG. 3;

FIG. 5A is an enlargement of FIG. 5 partially cut away;

FIG. 6 is a perspective view of an alternate tray with a lid eachfabricated from the present compositions with food items removed;

FIG. 7 is a perspective view of the alternate tray taken in thedirection of lines 7--7 of FIG. 6.

FIGS. 8-15 depict time/temperature response curves for ceramiccompositions exemplified in Examples 1-13.

SUMMARY OF THE INVENTION

The present invention provides ceramic compositions useful in theformulation and fabrication of microwave heating susceptors. The presentcompositions comprise a ceramic active microwave absorbing material anda binder.

The present microwave absorbing materials are common ceramic ingredientshaving a neutral lattice charge. The microwave absorbing materials cancomprise about 2 to 99.1% of the ceramic compositions. The bindersessentially comprise about 0.9 to 98% of the compositions. Conventionalbinder materials are suitable for use herein.

In its article aspect, the present invention resides in devicesfabricated from the present compositions. Such devices are microwaveheating susceptors generally in sheet form and which range in thicknessfrom about 0.5 to 8.0 mm. In a preferred embodiment, the heatingsusceptor is in the form of a tray. The susceptors find particularusefulness in disposable packages for the microwave heating of food.

Throughout the specification and claims, percentages are by weight andtemperatures in degrees Fahrenheit, unless otherwise indicated.

DETAILED DESCRIPTION OF THE INVENTION

In its composition aspect, the present invention relates to ceramiccompositions useful for fabrication into heating susceptors fordisposable packages for the microwave heating of food products. Thecompositions comprise a defined microwave absorbing material and abinder. In its article aspect, the present invention resides inmicrowave heating susceptor for packaged food items, to packages forsuch items and to the packaged food items themselves.

The microwave absorbing materials useful herein surprisingly include awide variety of ceramic materials previously regarded as microwavetransparent or used in ceramic compositions transparent to microwaves.By ceramic materials are meant materials comprising oxygen attached tonon-carbonaceous elements, and primarily to magnesium, sodium, calcium,iron, aluminum, silicon and mixtures thereof.

In the ceramic industry, a distinction is made between "greenware," aceramic composition before firing or vitrification, and the finished,fired or vitrified ceramic compositions prepared therefrom. The firingstep profoundly changes a large number of the ceramic composition'sproperties as the individual constituents are fused into a homogeneousmass. Broadly speaking, the present invention is directed towardcompositions which would be considered greenware in the ceramic arts.

Certain of the present microwave active materials have been used ingreenware ceramic compositions, but generally at marketedly differentconcentrations and for different purposes than in the present invention.For example, kaolin reduces plasticity and tends to make the greenwaremix short or lean. Likewise, alumina has a similar effect on theplasticity and will reduce green strength. Also, sodium metasilicate isnot used at levels greater than 1% since greater amounts cause stickingand hinder mold release properties as well as decrease green strength.

The present ceramic microwave absorbing materials and their othergeneral properties are well known and described generally, for example,in "An Introduction to the Rock Forming Materials," by Deer, Howie, andZussman, Longman Group Ltd., Essex, England, 1966 or in "The Potter'sDictionary of Materials and Techniques" by Frank and Janet Hamer,Watson-Guptill Publications (1986) and which is incorporated herein byreference. Materials as therein described are generally andconventionally classified as ortho and ring silicates, chain silicates,sheet silicates, framework silicates and non-silicates. However, thematerials useful herein can fall into any of these classificationsalthough not all materials in those classifications are useful herein.

As indicated above, the microwave absorbing materials useful hereinsurprisingly include a wide variety of ceramic materials previouslyregarded as microwave transparent. It is speculated herein that thesematerials have heretofore been unappreciated as being useful as consumermicrowave absorbing materials since most investigations of theirelectromagnetic interactions, i.e., absorption/transparency has beendone at very different frequencies or have been investigated as firedceramics. The present materials are further essentially characterized bya neutral lattice charge. "Neutral lattice charge" is used herein in itsconventional usage and means that the net relative electron surfacecharge densities of the material is essentially zero or that the cationexchange capability is essentially zero for the constituent chemicalmake-up of the ceramic material. The present ceramic materials arefurther characterized by relatively low electrical resistivity, i.e.,about 0.1 to 35 ohm.cm and are thus classifiable as semiconductors inthe broad sense of the term.

Exemplary specific materials include:

Sodium Metasilicate, Na₂ SiO₃ ;

Talc, Mg₆ [Si₈ O₂₀ ](OH)₄ ;

Kaolin, Al₄.[Si₄ O₁₀ ](OH)₈.4H₂ O;

Alumina and activated alumina, Al₂ O₃ ;

Clays (fine grained, natural, early argillareous materials);

Aluminosilicates;

Non-siliceous ceramics.

Of course, mixtures of these materials can also be used. Preferredmaterials include sodium aluminum silicate, clay, sodium metasilicateand kaolin and mixtures thereof due to the relatively flat or uniformityof their final heating temperature.

The present compositions include an effective amount of the abovedescribed microwave absorbing materials. The precise level will dependon a variety of factors including end use application, desired finaltemperature, and thickness of the susceptor to be fabricated from thepresent compositions. Good results are generally obtained when themicrowave absorbing material comprises from about 0.1% to about 98% byweight of the present ceramic compositions. Preferred compounds includefrom about 20% to 98% by weight of the microwave absorbing material. Forbest results, the ceramic compositions comprise about 40% to 98% byweight of the microwave absorbing materials. The particle size of themicrowave absorption material or refactory is not critical. However,finely ground materials are preferred inasmuch as the ceramic susceptorsproduced therefrom are smooth and uniform in texture.

Another essential component of the present ceramic compositions is aconventional ceramic binder. By the term "ceramic binder" is meant thatthe binder is capable of binding the present ceramic heating materialsinto a solid mass. The term is not meant to imply or require that thebinder material itself is necessarily ceramic in composition although itwell may be. Such ceramic binders are well known in the ceramic art andthe skilled artisan will have no problem selecting suitable bindermaterials for use herein. The function of the binder is to form theparticulate microwave absorbing material into a solid form or mass.Exemplary materials include both ceramic and plastic binders,respectively, such as cement, plaster of Paris, i.e., calcium sulphate,silica fiber, feldspar, pulverized Kelvar® (a polyamide fiber),colloidal silicas, fumed silicas, fiberglass, wood pulp, cotton fibers,and mixtures thereof. The binder can comprise from about 2% to 99.9% byweight of the present ceramic compounds, preferably from about 20% to80%. Exemplary, conventional plastic based binders, both thermoplasticand thermosetting, are described in U.S. Pat. Nos. 4,003,840 (issuedJan. 18, 1977 to Ishino et al. which is incorporated herein byreference.

In one preferred embodiment, the present compositions include binderswhich are organic thermoplastic resins especially those approved as foodpackaging material such as polyvinyl chloride, polyethylene, polyamides,polyesters, polycarbonates, polyimides, epoxies, etc. In theseembodiments, the thermoplastic resin binders can range from as little as20% up to 60% of the composition and preferably about 30% to 50%. Suchcompositions are especially well suited for fabrication into shapedmicrowave susceptors, especially food trays, e.g., for TV dinners orentrees.

In one highly preferred embodiment, the present ceramic compositionsadditionally desirably comprise common salt or sodium chloride as atemperature profile modulator. The temperature profile modulator canassist in reaching more quickly the final operating temperature of theceramic composition. Also, the salt increases modestly the finaloperating temperature of the ceramic composition. The preferred ceramiccompositions comprise from about 0.1% to about 6% by weight salt. Whileceramic compositions can be formulated having higher amounts of salt, noadvantage is derived therefrom.

The present ceramic compositions can be fabricated into useful microwaveheating susceptor articles by a simple admixture of the materials into ahomogeneous blend, and addition of sufficient amounts of water if neededto hydrate the binder. When plaster of Paris is used as the binder,typically, water will be added in a weight ratio to binder ranging fromabout 0.4 to 0.7:1. While the wet mixture is still soft, the ceramiccompositions can be fabricated into desirable shapes, sizes andthicknesses and thereafter allowed to harden or dry to a moisturecontent ranging from about 2.5% to 10%.

Of course, one advantage of the present invention is that upon heatingin a conventional microwave oven, e.g., 2450 MHz, the ceramiccompositions will relatively quickly (e.g., within 30 to 300 seconds)heat to a final temperature ranging from about 300° to 600° F. whichtemperature range is very desirable in providing crisping, and browningto foods adjacent thereto and consistent with safe operation of themicrowave oven.

Another advantage of the present ceramic compositions is that they canbe dried at temperatures above 180° F. Still another advantage of thepresent invention is that susceptors fabricated from the present ceramiccompositions provide a microwave field modulating effect, i.e., eveningout peaks and nodes, i.e., standing wave points and, it is believedindependent of wattage. This benefit is especially useful when sensitivefoods such as cookie doughs are being microwave heated.

Still another advantage of the present ceramic compositions is that theyare believed to be useful not only with microwave ovens operating at2450 MHz but at all microwave frequencies, i.e., above as low as 300MHz.

Another advantage of the present invention is that the ceramiccompositions can absorb oil and/or moisture from food items to bemicrowave heated, e.g., par-fried fish portions, without substantialadverse affect on heating performance.

It is important that the susceptors fabricated herein be unvitrified,i.e., not subjected to a conventional firing operation generally above800° F. to 1000° F. (426° C. to 538° C). Conventional firing can resultin a fused ceramic composition substantially transparent to microwaveand thus devoid of the desirable microwave reactive properties of thepresent invention.

The present ceramic compositions are useful in any number of microwaveabsorption applications. The present ceramic compositions areparticularly useful for fabrication into microwave susceptors which inturn are useful as components in packages for foods to be heated withmicrowaves.

For example, FIG. 1 illustrates generally a packaged food item 10fabricated in accordance with the teachings of the present invention andsuitable for microwave heating. FIG. 2 shows that the article 10 canoptionally comprise a six-sided outerwrap 12 which can be plastic, paperor other conventional packaging material such as the paperboard packagedepicted. The article can further comprise an inner assembly 14 disposedwithin the outerwrap 12 which can comprise a sleeve 16 fabricated from adielectric material (e.g., cardboard, paper, polyester) and disposedtherein a tray 18. In conventional use, the consumer will open thearticle 12, remove and discard the overwrap 12, and insert the entireassembly into the microwave oven. The sleeve 16 is helpful although notessential not only to prevent splattering in the microwave oven, butalso to assist in securing the food items against excessive movementduring distribution.

In FIG. 2, it can be seen that the sleeve 16 can comprise an opposedpair of open ends, 20 and 22, an upper major surface or top wall 24, alower major surface or bottom wall 26 and an opposed pair of minor sideor wall surfaces 28 and 30. As can be seen in FIG. 3, the tray 18 holdsor contains one or more food items 32. FIG. 4 shows the tray 18 with thefood items 32 removed. Disposed within the tray 18 is one or moremicrowave heating susceptors such as microwave susceptor heating panel34. In this preferred embodiment, the susceptors are generally flat orplanar and range in thickness from 0.020 to 0.250 inch.

Still referring to FIGS. 3 and 4, with the cooking of certain foods, itmay be desirable to heat the food items 32 from only or primarily oneside by use of the heating susceptor panel 34 while at the same timeminimizing the heating of the food item 32 by exposing it to microwaveradiation through the walls of the package assembly 14. To allowmicrowave radiation to reach the susceptor 34, the bottom wall 26 ismicrowave transparent at least to the extent that sufficient microwaveenergy can enter the package to heat the susceptor 34. Side walls 28 and30 an each optionally be shielded with shielding 29 as can top wall 24thereby restricting the entry of microwave radiation through these wallsto the food product as is known in the art. The shielding 29 can be ofany suitable type material of which aluminum foil is a currentlypreferred material. With the use of shielding, the microwave radiationpenetrates the microwave transparent bottom 26 only. Accordingly,cooking of the food product 32 in this embodiment is accomplishedsubstantially totally by the heat transferred to the food product 32from the susceptor 34 although some microwave entry through the openends 20 and 22 occurs. It is pointed out that the terms microwavetransparent and microwave shield are relative terms as used herein andin the appended claims.

In FIG. 5, it can be seen that the heating panel 34 can optionallycomprise a thin finish layer 36, e.g., 0.00005 to 0.001 inch (0.001 to0.025 mm) to impart desirable surface properties, e.g., color, waterrepellency, smooth appearance, stick free, etc. In the simplest form,such a layer can comprise ordinary paraffin or a sodium silicatepolymerized with zinc oxide. The finish layer does not substantiallyadversely affect the performance of the microwave susceptor. Suchsurface property modification finds particular usefulness when themicrowave susceptors are used in medical settings. For example, it isknown to fabricate surgical implants, e.g., discs, cylinders, fromferrites which absorb microwave radiation to thermally treat tumors. Insuch applications wherein the present compositions are employed, waterrepellency may be particularly desirable.

Other types of packages can be utilized with the ceramic microwaveheater compositions of the present invention. It is an importantadvantage that the present compositions can be fabricated intosusceptors of different configurations whether regular, e.g.,corrugated, or irregular.

Another embodiment is depicted in FIG. 6. Thermoplastic resins arepreferred for use as the binder materials. In this embodiment, thearticle 10 in addition to outerwrap 12 as shown in FIG. 2 can comprise amicrowave heating susceptor 40 fabricated into trays or shallow panswhether square, rectangular, circular, oval, etc. which serve both tocontain and heat the food items. Such tray shaped susceptors 40 findparticular suitability for use in connection with a batter type fooditem 44, especially cake batters or with casseroles, baked beans,scalloped potatoes, etc. In one particular embodiment the tray 40 canadditionally include a cover 42 also fabricated from the present ceramiccompositions. Trays 40 with covers 42 are especially useful for batterfood items like brownies in which it is desired to form an upper or topskin to the food item 44.

In still another embodiment shown in FIG. 5A, the panel susceptor 34 canadditionally comprise a backing layer(s), especially a metal foil, e.g.,aluminum 46. The foil serves to reflect back to the susceptor 34microwave energy passing through the susceptor 34. The incorporation ofa microwave shielding or reflecting layer 29 in close proximity on theopposite surface of the ceramic susceptor 34 also serves to act as asusceptor temperature booster to elevate the operating temperaturesubstantially above the temperature obtained without a microwaveshielding or reflective layer 29. Final temperature reached can be ashigh as 100° F. or more over similar structures without the metal foil.Also, the use of the temperature booster can reduce the need for athicker ceramic susceptor to obtain the same temperature therebyreducing both production costs as well as final weights of the microwavepackage. Since the ceramic compositions adhere to the metal foil withsome difficulty and cause an in heating interference due toconductor-wave phenomena interaction, it is preferable to treat thesurface of the metal foil with an intermediate or primer layer (notshown) for better adherency, i.e., ordinary primer paints, or to have anintermediate silicone layer, paper layer or other polymer layer, or toselect those binders for the ceramic compositions with increasedcapacity to adhere to metal foils.

The skilled artisan will also appreciate that the present compositionsabsorb microwave radiation at a wide range of frequencies and not merelyat those licensed frequencies for consumer microwave ovens.

The ceramic susceptor compounds of the present invention can also beutilized in non-disposable utensils adapted for a limited number ofrepetitive heating cycles by embedding the heating compositions orotherwise associating with a non-disposable utensil body. The susceptoris associated with the remainder of the utensil in a manner such that itwill be in heat transfer relation to a product to be heated in or on theutensil. The utensil can be in the form of an open top dish, griddle orthe like. However, the present compositions will exhaust their abilityto heat upon microwave exposure relatively quickly, i.e., after only afew cycles of operation.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative and not limitative ofthe remainder of the disclosure whatsoever. It will be appreciated thatother modifications of the present invention, within the skill of thosein the food arts, can be undertaken without departing from the spiritand scope of this invention.

EXAMPLE 1

Sodium metasilicate pentahydrate (100 grams) was mixed with 25 gramsdeionized distilled water, cast into a mat 5/32 inch (0.160 inch) thickand air dried overnight at 85° F. (29.4° C.). During drying the tileexhibited no shrinkage or breaking. Cast tile weight 3.5"×3.5"×5/32" was31.21 grams, density 0.992 g cm⁻³. The tile was subjected to a 750 watt,2460 MHz microwave field for a Period of five minutes while thetemperature of the tile surface was monitored using a Luxtron 750®Fluoroptic temperature monitor equipped with ceramic clad fiber optictemperature probes and interfaced with an IBM PC/AT computer for datacollection and handling. The recorded temperature profile of the tile isshown in FIG. 8 as line 1.

EXAMPLE 1A

Sodium metasilicate pentahydrate was mixed with sufficient distilledwater to form a cohesive mass (10% moisture). The mixture was compressedinto a disc 2.969 inches (7.540 cm) diameter and 0.160 inch thick. Thedisc weight was 22.44 grams, density 1.236 g cm⁻³. After air dryingovernight at 85° F. (29.4° C.) the temperature profile was determined asdescribed above in Example 1. The temperature profile of the tile is notshown but is very similar to line 1 in FIG. 8. The dielectric constantat 20° C. and 1000 MHz is 11.3 with a dissipation factor D or losstangent "tan δ" of 0.227.

EXAMPLE 2

100 grams of sodium metasilicate pentahydrate was mixed with 30 grams ofcalcium sulfate hemihydrate and after blending to a uniform mix 30 gramsof distilled water was added. The resulting mix was stiff and displayeda positive heat of reaction (exothermic). The mass was cast into tiles3.5"×3.5"×0.175 inches and air dried at 85° F. (29.4° C.) for 24 hours.The cast tile weight was 42.58 grams, density 1.212 g cm⁻³. The tile didnot display cracking or mold shrinkage. The tile was treated asdescribed in Example 1 with the recorded temperature profile shown asline 2 in FIG. 8. Weight loss upon heating was 29.76%.

EXAMPLE 2A

To the dry mix prepared in Example 2 was added 11.0 grams distilledwater so as to form a cohesive mass upon compression. The mixture wasthen compressed into a disc 3.00 inches (7.620 cm) diameter and 0.130inch thick. The disc weight was 19.98 grams, density 1.327 g cm⁻³. Afterair drying at 85° F. (29.4° C.) for 24 hours, the temperature profilesof the tile in a 2460 MHz microwave field was determined as described inExample 1. The temperature profile of the tile is similar to that shownas line 2 in FIG. 8. The dielectric constant at 20° C. at 1000 MHz is12.1 with a loss tangent "tan δ" of 0.125. Weight loss upon heating was19.9%.

EXAMPLE 2B Cast

100 grams of sodium metasilicate pentahydrate was mixed with 30 grams ofcalcium sulfate hemihydrate, 40 grams of Hawthorn Bonding Fireclay and40 grams of A. P. Green Fireclay. After blending to a uniform mix 210grams of distilled water was added. The resulting mix was plastic andeasily workable. The mass was cast into tiles 3.5"×3.5"×0.125 inches andair dried at 85° F. (29.4° C.) for 24 hours. The cast tile weight was31.61 grams, density 1.259 g cm⁻³. the tile did not display cracking ormold shrinkage. The heating structure was treated as described inExample 1 with the recorded temperature profile shown as line 2B in FIG.9.

EXAMPLE 2C Pressed

To 69 grams of dry mix as prepared in Example 2B was added 15.0 grams ofdistilled water. The resulting damp mix was compressed into a disc 3.00inches (7.620 cm) diameter and 0.135 inches thick. The disc weight was24.0 grams, density 1.530 gm cm⁻³. After drying in warm air at 85° F.(29.4° C.) for 24 hours, the temperature profile of the heater tile wasdetermined as previously outlined. The temperature profile of the tileis shown as line 2C in FIG. 9.

EXAMPLE 3A Cast

100 grams of calcined activated high alumina X-5111 (EnglehardCorporation, Edison, N.J. 08818) was dry blended with 40 grams ofmagnesium silicate (Ceramitalc HDT, R. T. Vanderbilt Company, Inc.,Norwalk, Conn. 06855). 65 grams of distilled water was added and aslurry prepared. The slurry was cast into 31/2 inch square tile frames0.125 inches thick and allowed to dry at 120° F. (48.9° C.) for 12hours. The resulting tile was cracked but exhibited minimal moldshrinkage. The tile was measured for heating performance in a microwavefield as previously detailed. The temperature profile of the heatingstructure is shown in FIG. 8 as line 3C. Weight loss upon heating was3.2%.

EXAMPLE 3B Pressed

A second dry mix was prepared as detailed above with 20 grams ofdistilled water. The resulting mix was compressed into a 3.00 inch(7.620 cm) disc, 0.125 inches thick with a density of 1.920 g cm⁻³.Evaluation for heating performance was made after drying at 120° F.(48.9° C.) for 24 hours. The heating profile is shown in FIG. 8 as line3P. Weight loss upon heating was 3.3%. The dielectric constant at 20° C.and 1000 MHz is 11.7 with a loss tangent "tan δ" of 0.172.

EXAMPLE 4 Cast

75 grams of calcined activated high alumina X-5111 (EnglehardCorporation) was dry blended with 75 grams of air floated kaolin #6 tile(Georgia Kaolin Company, Inc., Union, N.J. 07083) and 13 grams ofQ-Fiber Amorphous High Purity Silica Fiber (Johns-Manville, Denver,Colo. 80217). 84 grams of distilled water was added and a pasteprepared. The paste was cast into 3.5 inch square ×0.125 inch tiles anddried at 200° F. (93.3° C.) for 1 hour. The resulting tile was intactand displayed a 13.8% shrinkage. Tile weight was 25.99 grams, density1.201 g cm⁻³. The microwave performance of the heater tile is shown inFIG. 10 as line 4C. Weight loss upon heating was 2.3%.

EXAMPLE 4 Pressed

A second dry mix was prepared as detailed above with 24 grams distilledwater. The resulting mix was compressed into a 3.00 inch (7.620 cm)disc, 0.125 inches thick, density 1.833 g cm⁻³. Evaluation for heatingperformance was made after drying at 200° F. (93.3° C.) for 5 hours. Theheating performance is shown in FIG. 10 as line 4P. Weight loss uponheating was 1.5%. The measured dielectric constant at 20° C., and 1000MHz is 11.1 with a loss tangent "tan δ" of 0.147.

EXAMPLE 5

5.0 grams of sodium metasilicate pentahydrate, 30.0 grams calciumsulfate hemihydrate, 10.0 grams of calcined activated high aluminaX-5111 (Englehard Corporation), 35.0 grams Kentucky Clay #6(Kentucky-Tennessee Clay Company, Mayfield, Ky.), 50.0 grams Hexafil--asemi-reinforcing clay (Hammill and Gillespie, Inc., Livingston, N.J.)and 7.5 grams of Goldart--Cedar Heights air floated secondary clay(Minnesota Clay, Bloomington, Minn.) were dry blended together to auniform consistency. 62 grams of distilled water was added to the drypowder mix and a paste formed upon mixing. The paste was cast into 3.5inch square by 0.125 inch thick tiles and dried for 8 hours at 150° F.(65.6° C.). The resulting tiles were intact and displayed a 23.4%shrinkage upon drying. The tile weight was 27.58 grams, density 1.435 gcm⁻³. The microwave performance of the heater tile is shown in FIG. 10as line 5C.

A second dry mix was prepared as detailed above with 25.8 grams ofdistilled water added to the mix. The resulting mix was compressed intoa 3.00 inch (7.620 cm) disc, 0.125 inches thick, density 1.554 g cm⁻³.Evaluation for heating performance in a microwave field was made afterdrying at 150° F. (65.6° C.) for 8 hours. The measured heating profileis shown in FIG. 10 as line 5P.

EXAMPLE 5A Cast

A formulation similar to the one prepared in Example 5 was prepared withthe following modifications. 15 grams of calcined activated aluminaX-5111 (Englehard Corporation), 30 grams of Kentucky Clay #6(Kentucky-Tennessee Clay Company, Mayfield, Ky.) and 7.5 grams of YellowBanks #401 air floated clay (Minnesota Clay, Bloomington, Minn.) weredry blended with the other ingredients. 65 grams of distilled water wasadded to the dry powder mix and a paste formed upon mixing. The pastewas cast into 3.5 inch square by 0.125 inch thick tiles and air driedfor 8 hours at 150° F. (65.6° C.). The resulting tiles were intact andexhibited a 21.9% shrinkage upon drying. The tile weight was 27.21grams, density 1.388 g cm⁻³. The microwave performances of the heatertile is shown in FIG. 11 as line 5-1.

EXAMPLE 5B Pressed

A second dry mix was prepared as detailed above with 25.8 grams ofdistilled water added to the mix. The resulting mix was compressed intoa 3.00 inch (7.620 cm) disc, 0.125 inches thick, density 1.498 g cm⁻³.Evaluation for heating performances in a microwave was made after dryingfor 8 hours at 150° F. (65.6° C.). The measured heating profile is shownin FIG. 11 as line 5-2.

EXAMPLE 6

5.0 grams of sodium metasilicate, 30 grams calcium sulfate hemihydrate,15 grams of calcined activated high alumina X-5111 (EnglehardCorporation), 80 grams of Tennessee Clay #6 (Kentucky-Tennessee ClayCompany, Mayfield, Ky.) and 7.5 grams of Hawthorn Bonding Fireclay(Minnesota Clay, Bloomington, Minn.) were dry blended together to auniform consistency. 70 grams of distilled water was added to the drypowder and a paste formed upon mixing. The paste was cast into 3.5 inchsquare by 0.125 inch thick tiles and dried for 8 hours at 150° F. (65.6°C.), the resulting tiles were intact and displayed a 7.0% shrinkage upondrying. The tile weight was 28.34 grams, density 1.215 g cm⁻³. Themicrowave performance of the heater tiles is shown in FIG. 11 as line6C.

A second dry mix was prepared as detailed above with 26.0 grams ofdistilled water. The resulting damp mix was compressed into a 3.0 inch(7.620 cm) disc, 0.110 inches thick, density 1.694 g cm⁻³. Evaluationfor heating performance in a microwave field was made after drying at150° F. (65.6° C.) for 8 hours. The measured heating profile is shown inFIG. 12 as line 6P. As discernible from the shown profiles, a pressedembodiment in this example is preferable to the cast embodiment due tothe plateauing profile shape observed.

EXAMPLE 7

50 grams of sodium metasilicate pentahydrate, 30 grams of calciumsulfate hemihydrate, 10 grams of Hawthorn Bonding Fireclay and 50 gramsof sodium aluminum silicate were dry blended together to a uniformconsistency. 70 grams of the dry mix was added with stirring to 35 gramsof distilled water. The resulting paste was cast into a 3.5 inch squareby 0.125 inch thick tile and dried for 8 hours at 150° F. (65.6° C.).The tile exhibited no shrinking or cracking upon drying. The microwaveperformance of the heater tile is shown in FIG. 12 as line 7C.

To the remaining 70 grams of dry mix as prepared above, 13 grams ofdistilled water was added. The damp mix was compressed into a 3.0 inch(7.620 cm) disc, 0.110 inches thick, density 1.726 g cm⁻³. Evaluationfor microwave heating performance was made after drying at 150° F.(65.6° C.) for 8 hours. The measured heating profile is shown in FIG. 12as line 7P.

EXAMPLE 8

50 grams of Tennessee #6 Clay, 50 grams of Hawthorn Bonding Fireclay, 20grams of calcined activated high alumina X-5111 and 25 grams of sodiumaluminum silicate were dry blended to a uniform consistency. To 70 gramsof the dry mix was added 35 grams of distilled water, after mixing theresultant paste was formed into 3.5 inch square by 0.125 inch thicktiles and dried for 2 hours at 150° F. (65.6° C.). The tile displayed noshrinking or cracking after drying. Tile weight was 27.78 grams, density1.107 g cm⁻³. The microwave heating performance of the tile is shown inFIG. 9 as line 8C.

To the remaining 75 grams of dry mix prepared above was added 15 gramsof distilled water with mixing. The damp mix was then compressed intodiscs 3.00 inches (7.620 cm) diameter and 0.110 inches thick, density1.723 g cm⁻³. The discs were dried as described above and evaluated formicrowave heating performance in the usual manner. The heating curve isshown in FIG. 12 as line 8P.

EXAMPLES 9-13

Ceramic compositions were prepared having the compositions indicated inthe following table:

    ______________________________________                   Amount (grams)    Component        9      10     11    12   13    ______________________________________    Sodium metasilicate             5    Tennessee Clay #6       10     30    10    Hexafil                 10    X-5111 calcined bauxite 10     15         15    Hawthorn Bonding Fireclay                     20     10     15    30   25    A.P. Green Fireclay                     20     10           10    Goldart-Cedar Heights Clay                     20     10     15    20   20    Yellow Banks 401 20     10            5   10    Old Hickory Ball Clay   10      5     5    Redart Cedar Heights Clay                     20             5    Nytal ® Talc        10                25    Georgia Kaolin #6 Tile Clay                     20            10    50   25    Cornwall Stone          10    Gerstley Borate  20     10      5    Sodium aluminum silicate                            20     10         15    Feldspar                20            5    Kelvar ® Fiber (pulverized)                                   10    ______________________________________

Seventy grams of the above mixtures were each separately mixed with 35 gdeionized distilled water, individually cast or pressed into a mat 0.160inch thick and air dried overnight at 85° F. (29.4° C.). The heatprofiles are shown in FIGS. 13-15 with "c" indicating cast and "p"indicating pressed.

What is claimed is:
 1. A packaged food article to be heated by microwaveenergy in a microwave oven comprising:a tray for holding a food itemhaving a top and bottom surface, a substantially planar microwaveheating susceptor disposed within said tray, said microwave heatingsusceptor fabricated from a ceramic composition, comprising: a ceramicbinder; and a ceramic susceptor material which absorbs energy and havinga neutral lattice charge, wherein the compound is unvitrified, andwherein the susceptor is in intimate physical contact with the food itemand ranges in thickness from about 0.5 to 8 mm.
 2. The article of claim1 wherein the binder comprises about 2 to 99.9% by weight of thecomposition and wherein the ceramic susceptor material comprises about0.1 to 98% of the composition.
 3. The composition of claim 2 wherein theceramic composition additionally comprises 0.1% to 6% of sodiumchloride.
 4. The article of claim 3 wherein the ceramic susceptormaterial is selected from the group consisting of sodium metasilicate,talc, kaolin, calcined alumina, alumina or activated alumina, clay,aluminosilicates, sodium aluminosilicates and mixtures thereof.
 5. Thearticle of claim 4 wherein the binder is selected from the groupconsisting of calcium sulphate, cements, calcite, silica fiber, whetheramorphorus or crystalline, dolomite, aragonite, feldspar, pulverizedpolyamide fibers, colloidal silicas, fumed silicas, fiberglass, woodpulp, cotton fibers, thermoplastic resins and thermosetting resins. 6.The article of claim 5 additionally comprising:a sleeve fabricated froma dielectric material having a top major surface, a bottom major surfacespaced apart and parallel to the top surface, a pair of spaced, parallelwalls and a pair of spaced, opposite side openings and wherein disposedwithin which sleeve is the tray.
 7. The article of claim 6 wherein thetray comprises a tray bottom wall and a side wall,and wherein thesusceptor conforms to the shape of the tray bottom wall and is disposedabove the tray bottom wall.
 8. The article of claim 7 additionallycomprising a food item disposed within ne tray on top of the susceptor.9. The article of claim 8 wherein the article additionally comprises asecond heating susceptor disposed within the tray spaced apart andparallel to the first susceptor, said second susceptor overlaying thefood item and in physical contact therewith.
 10. The article of claim 7or 8 wherein the tray is circular.
 11. The article of claim 7 or 8wherein the tray includes a plurality of side walls at least two ofwhich are parallel and of equal size and wherein the first and secondsusceptors are compositionally similar.
 12. A packaged food article tobe heated in a microwave oven,a microwave heating susceptor in the formof a tray for holding a food item; wherein the susceptor is capable ofheating in a microwave oven, and wherein said susceptor is fabricatedfrom a ceramic composition, comprising: a ceramic binder; and a ceramicsusceptor material which absorbs energy and having a neutral latticecharge, wherein the compound is unvitrified.
 13. The article of claim 12wherein the binder comprises about 2 to 99.9% by weight of thecomposition and wherein the ceramic susceptor material comprises about0.1 to 98% of the composition.
 14. The composition of claim 13 whereinthe ceramic composition additionally comprises 0.1% to 6% of sodiumchloride.
 15. The article of claim 14 wherein the ceramic susceptormaterial is selected from the group consisting of sodium metasilicate,talc, kaolin, calcined alumina, alumina or activated alumina, clay,aluminosilicates, sodium aluminosilicates and mixtures thereof.
 16. Thearticle of claim 15 wherein the binder is selected from the groupconsisting of calcium sulphate, cements, dolomite, calcite, silicafiber, whether amorphorus or crystalline, aragonite, feldspar,pulverized polyamide fibers, colloidal silicas, fumed silicas,fiberglass, wood pulp, cotton fibers, thermoplastic resins andthermosetting resins.
 17. The article of claim 16 wherein the ceramicsusceptor material is an aluminosilicate.
 18. The article of claim 17additionally comprising:a sleeve fabricated from a dielectric materialhaving a top major surface, a bottom major surface spaced apart andparallel to the top surface, a pair of spaced, parallel walls and a pairof spaced, opposite side openings and wherein disposed within whichsleeve is the tray.
 19. The article of claim 18 wherein the traycomprises a tray bottom wall and a side wall,and wherein the susceptorconforms to the shape of the tray bottom wall and is disposed above thetray bottom wall.
 20. The article of claim 19 additionally comprising afood item disposed within the tray.